I have an LG V35. I'm using Neutron for my player. Playing back a 1 KHz sine wave at any bit/sampling rate with no load on the output, I get 1.03 V-RMS out with the the quad-DAC on, and .7 V-RMS with it off - same as Amir. Does this mean I'm being limited to 16 bit audio, as Amir suggests? I intend to use the phone to drive a headphone amp, and would like to get the output right.
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Update... I figured out how Amir got 2.0 V-RMS. In "High Impedance Headphones" mode, you get 2.0 V-RMS. in Auxiliary mode, which is what you get when you plug your phone into a headphone amp, you get 1.0 V-RMS. In order to get that "High Impedance Headphones" mode, you have to load the output of the phone with 50 to 600 ohms, on each of the two channels. Any higher, you get Auxiliary mode. Any lower, you get the regular headphones mode.
This means something. Those of you plugging in an adapter to trick the phone into high impedance mode aren't getting it. You're getting auxiliary mode, which is 1 V-RMS. If you want high impedance mode (2 V-RMS), you have to make an adapter with a load on each channel, then disconnect the load after you get your headphones connected - kind of difficult...
WARNING... Tricking your phone into either high impedance mode or auxiliary mode when using low impedance headphones forces your phone to output way more power than it was designed for. You are risking blowing your phone's headphone output chip.